As a kind of indoor flooring material, wooden floor is widely used. Such wooden floor is generally assembled by coupling a plurality of smaller size floor panels together. It is known that such floor panels can be coupled in various ways. According to a first possibility, the floor panels are attached on the underlying floor or support frame, either by gluing or by nailing them on. This technique has a disadvantage in that it is rather complicated and that subsequent changes can only be made by breaking out the floor panels. According to a second possibility, the floor panels are installed loosely onto the sub-flooring, whereby the floor panels mutually match into each other by means of a tongue and groove coupling. For example, CN02803650.6 discloses a rectangular floor panel having coupling devices in form of tongues and grooves. Such floor panel comprises an undercut groove on one long side and a projecting tongue on an opposite long side of the floor panel. The undercut groove has a corresponding upward inner locking surface at a distance from its tip. Tongue and undercut groove are formed to be brought together and pulled apart by pivoting motion with a center close to the intersection between the surface planes and the common joint plane of two adjoining floor panels. Such floor panel has an advantage in that it is much cheaper and convenience to install and repair. But a disadvantage of such floor panel is that an unreasonable design of the tongue and groove possibly results in that the floor panels cannot be coupled tightly when gaps between the floor panels or bumps on the coupling surfaces occur. These defects not only affect the appearance and use of the floor covering but shorten the lifespan of the floor covering.